Let me tell you a bed time story.
Many people use trichlor and end up with CYA levels that are too high for their FC level. Their pools turn green. They go to the pool store and are sold algaecide to combat the green and another bucket of trichlor and several bags of shock. This goes on and on and on until one day they get fed up with it all and start looking on the internet and find a site called Trouble Free Pool :-D and get their pool under control and they live happily ever after singing the praises of BBB! :wink:
Though it is certainly true that chlorine alone can prevent algae growth, there are some situations where an algaecide can be useful. One is as an insurance policy to prevent algae growth in case one forgets to add chlorine. A small weekly maintenance dose (or partial maintenance dose) of PolyQuat 60 will prevent the pool from turning into a swamp if you forget to add chlorine. Again, it's not necessary, but can be seen as an insurance policy (at a price of around $2-3 per week depending on pool size). In fact, one of the benefits of having 30-50 ppm Borates in the water is as an algaecide, but of course in this case you don't have to keep spending money each week since the Borates just sit there helping to prevent algae growth.
Another use for algaecide is when the phosphate level is extraordinarily high (well over 3000 ppb) in which case it may not be worth fighting or keeping away algae if the chlorine loss rate is exceptionally high. In this case, an expensive phosphate remover can be used. This is a very rare situation, however.
Now that we know about The Liquidator, there really is no excuse for using an algaecide as insurance since if one is concerned about forgetting to dose or being out of town, one can now have automated dosing of chlorinating liquid or bleach.
The most common algaecide being sold to unsuspecting consumers is the linear quat kind (it foams!), the second most common is copper sulfate (it's REALLY cheap to make so it's very high profit and the consumers like it because it lasts for 3 months!)
NEITHER are needed with the freqeuncy they are being sold and recommened. They are being sold as NECESSARY pool chemicals and not sometimes needed additives. There are some (actually very few) cases where an algecide is needed but, IMHO, if they are needed on a regular and longterm basis there are some problems with the water chemistry that need to be addressed (such as CYA levels that are extremely high and STILL continuing to use trichlor). Problem is the pool stores will lose all those lucrative trichlor and algaecide sales if they tell their customers how to fix their pools.
I keep my water very warm (90 or 92 degrees in the Summer) and my chlorine levels relatively low (@3 ppm at 40 ppm cya). I used to start to get some algae growth in the crevices of the deep end in late Summer. I find that a maintenance dose of the poly 60 type algaecide keeps any algae from gaining a foothold. Of course, I could also probably boost my chlorine to 5ppm.
The use of Borates to 50 ppm is also a very effective algaestatic and the plus side is you don't have to keep dosing it all the time so it becomes very cost effective. It also makes your water feel 'silky' and makes it 'sparkle' (I know these are supbjective but they have been reported over and over again by people that had added borates to their pool.) Oh, I almost forgot. It also helps stabilize your pH!
But you are overlooking the other benefits of borax that you don't get from algaecide, such as the increased pH stability and better feeling water. 80 lbs would be less than 18 boxes @2.99/box is less than $54 dollars. (I will even throw in the less than 4 galloons of acid needed and use a high price of $5 gal for an additional $20. We are now well under less than $74 and in reality probably much closer to $60). With a 32 oz bottle of polyquat 60 going for usually between $18-$25 per quart and needing a dose of about 1 pt per month the borax soon becomes much more cost effective! It stays in your water except what is lost by backwashing and splashout and has a wide active range (30-50 ppm for chlorine/bromine, 50-80 ppm for biguanide). The initial dosing is the main expense. Once it's in the water it stays pretty stable for a long period. I have only had to bump mine up ONCE in a three year period and a big part of that was because of a leak.